Array descriptor address (PB9)

[Christopher Carroll]

In 2005, Eric Cochran posted a thread called Yes, You CAN Redim Arrays Via A Ptr To Their Descriptor... in the source code forum.

A reply to the initial message implied that this could be a mistake.

However, in the PB8 and PB9 help, it states:

VARPTR
VARPTR may also be used to locate an array descriptor, as well as the array data itself. To find the address of an array descriptor, use the array name with empty parentheses: VARPTR( x( ) ).

and

REDIM - Restrictions
When a REDIM statement is executed, the location of the array elements always moves in memory; however, the array's Descriptor location (VARPTR(arrayname()) will remain fixed at the original location.

I have found this to be a useful technique (i.e. passing array data and meta-data to generic forms) and have had no problems using this. However, I am interested if anyone else has found related issues.

To obtain the array descriptor address, use the following syntax as per the help text:

    'Local hPtr As <type> Ptr
    'hPtr = VarPtr(MyArray())

    ' e.g.
    Local MyArray() As String
    Local hPtr As String Ptr
    ReDim MyArray(5)
    hPtr = VarPtr(MyArray())

Here is a sample of its use, using a string array. It works with other data types, including UDT's.

' test array descriptor address. Compiler: PB9

#Compile Exe
#Dim All

Function TestStr(aData() As String) As Long
' test string array descriptor
' aData(): parameter is ByRef, but pass array address descriptor value as ByVal
' return:  upper bound of array
' e.g.:    Call TestStr(aStrData())      passed array ByRef
' or:      Call TestStr(ByVal hStrPtr)   passed address of array descriptor
'                                        for array ByVal
    Local lUB As Long

    lUB = UBound(aData)
    ReDim Preserve aData(lUB + 2)
    aData(lUB + 1) = "apple"
    aData(lUB + 2) = "orange"
    Function = UBound(aData)
End Function

Function PbMain()
    Local aStrData() As String
    Local hStrPtr As String Ptr

    ReDim aStrData(4)
    Array Assign aStrData() = "cat", "dog", "rat", "bat", "eel"
    ? "1. Upper bound of string array is: " + Str$(UBound(aStrData)) + $crlf + _
        "with value: " + aStrData(UBound(aStrData)) + $crlf + _
        "elements: " + Str$(ArrayAttr(aStrData(), 4))
    hStrPtr = VarPtr(aStrData())
    Call TestStr(ByVal hStrPtr)         ' same as: Call TestStr(aStrData())
    ? "2. Upper bound of string array is: " + Str$(UBound(aStrData)) + $crlf + _
        "with value: " + aStrData(UBound(aStrData)) + $crlf + _
        "elements: " + Str$(ArrayAttr(aStrData(), 4))
End Function

Other references include:
A Technique For Using Dynamic Arrays In UDTs

and
storing dynamic arrays inside a UDT

Christopher

[Michael Mattias]

FWIW..

FUNCTION RedimMyArray (Z() AS anthing, newubound AS LONG) AS LONG
  REDIM  Z(newUbound) 
END FUNCTION

.... is redimming an array via a pointer to its descriptor, too.

It basically HAS to work. It's also the basis for....
Single Function to process any array type (CC3/Win7+) 12-17-03

There's only one way it WON'T work, and that should not occur unless you are using a mixture of PB/DLL 5/6x with PB/Win 7+ AND you are passing a UDT array.